RTP system technologies have been developed to increase manufacturing throughput of wafers while minimizing their handling. The types of wafers referred to here include those for ultra-large scale integrated (ULSI) circuits. RTP refers to several different processes, including rapid thermal annealing (RTA), rapid thermal cleaning (RTC), rapid thermal chemical vapor deposition (RTCVD), rapid thermal oxidation (RTO), and rapid thermal nitridation (RTN).
The uniformity of the process over the surface of the substrate during thermal processing is also critical to producing uniform devices. For example, in the particular application of complementary metal-oxide-semiconductor (CMOS) gate dielectric formation by RTO or RTN, thickness, growth temperature, and uniformity of the gate dielectrics are critical parameters that influence the overall device performance and fabrication yield. Currently, CMOS devices are being made with dielectric layers that are only 60-80 .ANG. (10.sup.-10 m) thick and for which thickness uniformity must be held within a few percent. This level of uniformity requires that temperature variations across the substrate during high temperature processing cannot exceed a few degrees Celsius (.degree. C.). Therefore, techniques that minimize temperature non-uniformity are very important.
In one RTP process, wafers are loaded into a processing chamber at a temperature of several hundred .degree. C. in a nitrogen (N.sub.2) gas ambient atmosphere. The temperature of the wafer is ramped to reaction conditions, typically at a temperature in the range of about 850.degree. C. to 1200.degree. C. The temperature is raised using a large number of heat sources, such as tungsten halogen lamps, which radiatively heat the wafer. Reactive gases may be introduced before, during, or after the temperature ramp. For example, oxygen may be introduced for growth of silicon dioxide (SiO.sub.2).
As mentioned, it is desirable to obtain temperature uniformity in the substrate during processing. Temperature uniformity provides uniform process variables over the substrate (e.g., layer thickness, resistivity, and etch depth) for various process steps including film deposition, oxide growth and etching.
In addition, temperature uniformity in the substrate is necessary to prevent thermal stress-induced wafer damage such as warpage, defect generation and slip. This type of damage is caused by thermal gradients which are minimized by temperature uniformity. The wafer often cannot tolerate even small temperature differentials during high temperature processing. For example, if the temperature differential is allowed to rise above 1-2.degree. C./centimeter (cm) at 1200.degree. C., the resulting stress is likely to cause slip in the silicon crystal. The resulting slip planes will destroy any devices through which they pass. To achieve that level of temperature uniformity, reliable real-time, multi-point temperature measurements for closed-loop temperature control are necessary.
One way of achieving temperature uniformity is by rotating the substrate during processing. This removes the temperature dependence along the azimuthal degree-of-freedom. This dependence is removed since, as the axis of the substrate is collinear with the axis of rotation, all points along any annulus of the wafer (at any arbitrary radius) are exposed to the same amount of illumination. By providing a number of pyrometers and a feedback system, even the remaining radial temperature dependence can be removed, and good temperature uniformity achieved and maintained across the entire substrate.
One example of a type of mechanical rotation system in use today is shown in FIG. 1. This type of system is similar to those used and sold today by Applied Materials, Inc., of Santa Clara, Calif. Certain details of such systems are provided in U.S. Pat. No. 5,155,336, entitled "Rapid Thermal Heating Apparatus and Method", issued Oct. 13, 1992, assigned to the assignee of the present invention, and incorporated herein by reference. In this type of mechanical rotation system, the substrate support is rotatably mounted on a bearing assembly that is, in turn, coupled to a vacuum-sealed drive assembly. For example, FIG. 1 depicts such a system. A wafer 12 is placed on an edge ring 14, which is in turn friction-fit on a cylinder 16. Cylinder 16 sits on a ledge of an upper bearing race 21 which is magnetic. Upper bearing race 21 is disposed within well 39 and revolves, by virtue of a number of ball bearings 22 (only one of which is shown), relative to a lower bearing race 26. Lower bearing race 26 is mounted generally at a chamber bottom 28. A water-cooled reflector 24 is positioned on chamber bottom 28 as part of the temperature measuring system (details of which are not shown). Magnet 30 is located adjacent the portion of chamber bottom 28 opposite upper magnetic bearing race 21. The magnet is mounted on a motor-driven magnet ring 32.
Magnet 30 is magnetically coupled to magnetic bearing race 21 through chamber bottom 28. By mechanically revolving magnet 30 about the central axis of chamber bottom 28, upper bearing race 21 may be made to rotate as it is magnetically coupled to magnet 30. In particular, torque is transferred to upper bearing race 21 from motor-driven magnet ring 32. The rotation of upper magnetic bearing race 21 results in the desired rotation of wafer 12 through cylinder 16 and edge ring 14.
While fully capable of accomplishing the intended function, the above system has some disadvantages. For example, it is commonly seen that the sliding and rolling contact associated with ball bearings leads to particle generation in the processing chamber. This particle generation arises from the contact between the ball bearings and the races as well as from the necessary use of lubrication for the bearing system.
As another example, the bearing and race system requires a complicated bearing structure with many low-tolerance interconnections. These interconnections result in a large amount of surface area available for the adsorption of undesirable gases and vapors.
Another disadvantage occurs when resort is made to complicated rotational mechanisms. When the objectives of smoother and faster rotations are met using complicated rotational mechanisms, the complicated mechanisms are often damaged by the reactive process gases in other portions of the chamber. This is because these mechanisms are often particularly delicate, e.g., with many low-tolerance interconnections, and are unable to withstand the corrosion and other damage caused by hot process gases.
A related disadvantage occurs when gaseous products of the chemical reactions on the wafer are not fully exhausted via a pumping system. Some amount of these gases may escape the pumping system and undesirably flow to regions below the plane of the wafer. For example, a typical silicon deposition may occur by the reaction of trichlorosilane (TCS) and molecular hydrogen (H.sub.2) in a processing region above the wafer. These reactive gases may deleteriously affect certain portions of the processing chamber.
Regions which may be so affected include the region forming the well containing the bearing/race system. Many of the sensitive components relating to rotation may be located in this well. In particular, damage and corrosion may be caused to the bearings and the exterior of the cylinder by the presence of hot gases in these regions.
Another problem associated with present rotation systems is the occurrence of eccentricities in the rotation. For example, as in FIG. 1, it is usual to have a rotationally-driven assembly such as race 21 support an intermediate cylinder 16 which in turn supports a wafer 12 via edge ring 14. In present systems, if intermediate cylinder 16 is not adequately secured to driven assembly 21, intermediate cylinder 16 may spin in an eccentric manner, particularly if its connection to rotationally-driven assembly 21 is non-circularly symmetric. In other words, if intermediate cylinder 16 is held in a non-circularly-symmetric frame, it tends to have an eccentric rotation, especially at high rotational speeds.
Another disadvantage concerns cleaning and repair. Complicated bearing and race rotation systems are difficult to dismantle and clean. For example, it is difficult to dismantle and individually clean numerous ball bearings in such a system.
Therefore, it would be usefull to provide a magnetic levitation drive that is easy to repair, has a relatively uncomplicated structure which is easy to dissemble, and which provides high speed, stable, and smooth rotations.